Call of Juarez came out not long ago, and is a beautiful game, besides being one of the best shooters I've seen in a LONG time. Be warned though, it's beauty is very intensive. You'll need to crank down the eye candy unless you've got a nice rig.
Yes, there are other cowboy games, but none of them are really good.
There is also a decent third-person shooter/free-range game called GUN that was rather good. However, its violence and story content was rather adult-oriented, more than most games. If it's for a kid, I'd steer clear on that one, but it's all in how you raise your kid.